Preheat oven to 350°F (175°C).
In a skillet, brown ground beef with onion over medium heat until beef is fully cooked and onion is soft. Drain excess fat.
In a large bowl, whisk cream of mushroom soup with milk. Season with salt and pepper.
Stir beef mixture into the soup mixture, then spread evenly in a 9x13-inch baking dish.
Sprinkle cheddar cheese over the beef layer.
Arrange frozen tater tots in a single layer over the cheese.
Bake for 45–50 minutes, or until tater tots are golden brown and crispy.
Let rest for 5 minutes before serving.
